Engineering geological investigation of some unstable and stable sections of the Ibadan-Iwo-Osogbo highway was undertaken to unravel the intrinsic reasons responsible for continuous pavement failure along this road. Eighty disturbed and forty undisturbed soil samples were collected at different depths from twenty test pits of six selected failed sections (FS) and two stable sections (SS). Road construction analysis was done on these soil samples using standard methods. Liquid limit (22 - 64% and 32 - 40%), plasticity index (13 - 41% and 12 - 18%) and percentage fines (47 - 59% and 32 - 41%) indicated fair to poor and fair to good subgrade materials of FS and SS respectively. Medium to high plasticity and high clay content of soils of FS are indicative that the soils possess medium to high swelling potential. The activity of clay in soils of FS was approximately 0.3 – 1.2 and classified as inactive to normal clay. High linear shrinkage (> 10%), low compacted density and predominance of fines in soils of FS caused the instability. Exposure to excessive moisture led to strength reduction of the soils. Triaxial compression showed cohesion (72.6 - 127.0 kN/m2) and internal friction (12.7° - 33.3°) indicating moderate to good shearing strength of the soils. A coefficient of compressibility of 0.1 – 0.5 kN/m2 indicated incompetency of the soils for road construction and the coefficient of consolidation was 0.01 – 0.30 m2 /year. The subsoil of the FS is impervious with a relatively low coefficient of permeability, indicative high saturation in the region. Thus, the pavement failure was caused by water absorbing clayey soil, poor geotechnical parameters of the soils and a poor drainage network. ; Kako bi se otkrili razlozi neprekidnoga lomljenja i pucanja kolnika duž ceste Ibadan-Iwo-Osogbo, napravljena su inženjerskogeološka istraživanja i nestabilnih i stabilnih odsjeka ceste. Uzeto je 80 poremećenih i 40 neporemećenih uzoraka tla s različitih dubina iz 20 testnih bušotina smještenih na 6 razlomljenih i 2 stabilna odsjeka. Načinjena je analiza konstrukcije ceste uporabom tih uzoraka i standardnih metoda. Vrijednosti granice tekuće faze (22 – 64 i 32 – 40 %), indeks plastičnosti (13 – 41 i 12 – 18 %) te postotak finoće (47 – 59 i 32 – 41 %) uputili su na solidan do loš stupanj za razlomljene i solidan do dobar stupanj za stabilne odsjeke. Srednji do velik indeks plastičnosti i znatan udio gline u tlu na razlomljenim dijelovima pokazali su kako takvo tlo ima srednji do visok potencijal bubrenja. Glina u tim tlima imala je aktivnost 0,3 – 1,2 te je klasificirana kao neaktivna do stabilna glina. Velik iznos linearnoga skupljanja (>10 %), mala gustoća (kompakcija) i prevladavanje sitnozrnatoga detritusa uzrokom je nestabilnosti. Izloženost velikoj vlazi uzrokovala je smanjenje čvrstoće tla. Trosna kompresija uputila je na koheziju (72,6 – 127,0 kN/m2 ) te unutarnje trenje (12,7 – 33,3 o ) pokazujući umjerenu do dobru smičnu čvrstoću takva tla. Koeficijent kompresibilnosti (0,1 – 0,5 kN/m2 ) uputio je na nepogodnost tla za gradnju prometnice, kao i koeficijent konsolidacije (0,01 – 0,03 m2/god). Tlo je ispod razlomljenih odsjeka nepropusno s relativno malim koeficijentom propusnosti te visokim (regionalnim) zasićenjem. Dakle, lomovi su uzrokovani apsorpcijom vode u glinovito tlo, a time i slabim geotehničkim parametrima, odnosno slabom drenažom.